1. Field of the Invention
The present invention relates generally to the treatment of exhaust gases for discharge to the atmosphere, and more particularly to methods and apparatus for treating and recovering energy from hot exhaust gases.
Exhaust gases suitable for treatment by the system of the present invention include combustion exhaust gases produced in fuel burning furnaces, roasters and the like, exhaust gases such as those produced in cement kilns and the like, and exhaust gases containing such components as nitrogen, carbon dioxide, carbon monoxide, hydrogen chloride, hydrogen sulfide, hydrocarbon gases, and the like. The exhaust gases are preferably essentially inert but include noxious components and traces of combustible gases.
2. Prior Art
Hot exhaust gases generated during the combustion of fuel have commonly been disposed of by exhausting them to atmosphere through tall chimneys or stacks. Disadvantages of this method of disposal include resulting air pollution and its harmful effects on the environment, a waste of recoverable heat energy, and the high cost of constructing and maintaining tall stacks. Loss of recoverable heat energy is unavoidable because gases discharged into a stack must be substantially hotter than ambient air to produce an up-draft in the stack and to avoid condensation in the chimney. Moreover, the latent heat of steam in flue gases in not generally recovered in order to avoid condensation and the attendant corrosion, as a result of which additional, available heat energy is being wasted.
Where the latent heat of steam is not recovered, the system designer must work with "low heating values" of the fuels rather than "high heating values". Low and high heating values for fuels are given in such handbooks as the John N. Perry Engineering Manual, published in 1959 by McGraw Hill, where the following typical heating values are given:
______________________________________ High Heating Low Heating Gas Value Value ______________________________________ Hydrogen 60,958 Btu/lb 51,571 Btu/lb Methane 32,861 Btu/lb 21,502 Btu/lb Methyl alcohol 10,270 Btu/lb 9,080 Btu/lb (vapor) ______________________________________
As will be apparent from these heating values, about 18 percent more Btu/lb can be recovered from hydrogen if its high heating value can be utilized, about 11 percent more from methane, and about 13 percent more from methyl alcohol vapor. Prior systems have not been able to utilize the high heating value of such gases.
As the public concern about air pollution has increased, stack heights have been increased to affect better dispersion of pollutants. However, increasing stack height adds to the cost of constructing and maintaining stacks, yet provides no solution to the underlying problem, i.e., avoiding emission in the first instance of harmful substances such as sulfur oxides, chlorine gases, phosphor oxides, etc.
A significant factor in air pollution is the increasing level of gaseous airborne pollutants which combine with moisture in the air to produce acids, e.g. carbon dioxide, sulfur dioxide, chlorine and fluorine. The carbon dioxide content in some industrial districts is as high as ten times normal. Acid forming pollutants have been found in some instances to increase the acidity of rainwater from its normal pH of about 6.9 to values of 4.0. Rainwater having a pH of 5.5 or less will destroy aquatic life and can do substantial harm to buildings, monuments, and other structures.
One proposal for removing acid forming components from exhaust gases is to scrub the entire flow of exhaust gases with water and caustic prior to discharging them through a stack. However, scrubbing the entire exhaust gas flow requires large quantities of water, which are not always available, and requires costly, large capacity scrubbing equipment. Indeed, scrubbing the entire flow of exhaust gases from some incinerators requires at least half the amount of water, by weight, of the solid wastes burned in the incinerator. Treating the large volume of scrub water needed in such a process is very costly and contributes to the impracticality of scrubbing as a total solution to the acid pollutant problem.
Another difficult pollutant to deal with effectively is sulfur in the flue gases. One proposal for the desulfurization of flue gas utilizes a series of heat exchangers to extract heat energy from the flue gas prior to a scrubbing operation. Heat extracted from the gas is returned to the gas following desulfurization and the gas is exhausted through a tall stack for diffusion into the atmosphere. This proposal has the disadvantages of wasting heat energy recovered from the gases, requiring large volumes of scrubbing water, requiring the use of a tall stack, and polluting the air with such noxious components as are not removed during scrubbing.
The problem of disposing of exhaust gases is now recognized as a major concern in industrial countries throughout the world. Dispersing emissions through the use of tall stacks is no longer regarded as an acceptable solution. Applicant's U.S. Pat. No. 3,970,524 discloses a system for gasification of solid waste materials and a method for treating the resulting gases to produce commercially useable gases in such a manner that dispersion through stacks is not necessary. A feature of one embodiment of this patent is pressurization of a combustion zone to such pressures as will permit blower and/or compression units to be eliminated from the gas treatment system. Another feature is the use of a multichamber gas treatment unit in which noxious gas components are sublimed or "frozen out" and thereby separated from the clean useable gas components. A problem not addressed by U.S. Pat. No. 3,970,524 is that of providing a system for treating combustion exhaust gases and productively reclaiming heat energy from the hot gases. This problem is, however, dealt with in applicant's U.S. Pat. No. 4,126,000 which teaches reclamation of heat energy by the transfer of the sensible and latent heat of the gases to a power fluid in indirect heat exchange relationship therewith, as in a conventional heat exchanger. However, the economics of indirect heat exchange at the lower temperature levels are very poor and reduce the over-all desirability of such a system. Applicant's U.S. Pat. No. 4,265,088, discloses a system which utilizes direct heat exchange between the hot gases and a power fluid to improve the economics and thermal efficiency of the system.
Notwithstanding the improvements in exhaust gas pollutant control and heat reclamation economics made possible by the systems disclosed in applicant's prior patents and copending application, a major problem not dealt with is the thermal inefficiency resulting from use of conventional combustion or other gas producing systems. A large amount of available power today is derived from fossil fuel fired furnace units which provide the thermal energy for steam generation in boiler units. In a conventional steam generating boiler system, preheated feed water is treated in a series of heat exchange sections to ultimately produce steam at the desired temperature and pressure for driving power generating steam turbines and the like. The boiler feed water is typically converted to high temperature, high pressure steam by initial heating in an economizer section, by subsequent passage through various superheater sections, often through a reheater section and subsequently through boiler convection and radiation sections. The fossil or manufactured fuel fired to produce the thermal energy which is transferred to the boiler feed water to produce the high temperature and pressure steam is converted to a hot exhaust gas which typically exits the boiler through an air preheater as its final stage. In this final stage, combustion gases having temperatures of about 300.degree.-350.degree. C. exchange their thermal energy with compressed ambient air with the result that the gases exhaust the unit at about 130.degree. C. to 180.degree. C. and the air is heated to about 200.degree. C. The 130.degree. to 180.degree. C. exhaust gas is further processed to separate pollutants and reclaim heat values while the heated air is utilized, serving, for example, as the combustion air fed to the boiler or combustion unit. Air preheaters are well known to require from 60% and 70% of the boiler's heat exchange surface area and to operate at thermal efficiencies in the 50-60% range. See, Hicks, Standard Handbook of Engineering Calculations (1972). Accordingly, if the preheater could be eliminated without a corresponding loss in heat reclamation capacity, a substantial cost and energy savings could be achieved.